A detergent-insoluble, actin-containing matrix is associated with the inner surface of the plasma membrane of murine tumor and lymphoid cells. Morphological and biochemical studies have provided strong evidence that this matrix forms a membrane "skeleton" extending over the entire inner face of the plasma membrane. The matrix consists of seven major protein components. Four of these (70k, 69k, 38k, and 36k) are released from the matrix by treatment with EGTA and have been shown to be peripheral proteins associated with the cytoplasmic face of the membrane. The major structural components of the matrix are proteins of 20k and 40k molecular weights, which have the unusual property of not entering SDS-polyacrylamide gels during electrophoresis. These two proteins account for about 80% of the matrix and about 15% of total plasma membrane protein. Their solubility properties indicate that they are integral-membrane proteins. About half of the plasma membrane-associated actin is tightly associated with the skeleton, where it may act as a site for attachment of actin filaments to the membrane. (A)